This invention relates to NMR resonators. More particularly, this invention relates to a method of optimizing birdcage and related resonators for high Q factor, as well as birdcage resonators thus optimized.
It has been known to structure a radio-frequency resonator coil in a birdcage form in order to obtain a highly homogeneous magnetic field as described, for example, in U.S. Pat. No. 4,694,255 issued Sep. 15, 1987 to C. Hayes and xe2x80x9cExperimental Design and Fabrication of Birdcage Resonators for Magnetic Resonance Imagingxe2x80x9d (T. Vullo, et al., Magnetic Resonance in Medicine, 24, 243 (1992)). Birdcage resonators are so called because of their general structure having a pair of generally circular conductive elements (the xe2x80x9cringsxe2x80x9d) separated in a longitudinal direction to define the axis of the resonator and a plurality of conductive segments (the xe2x80x9crungsxe2x80x9d) evenly spaced about the circumference of and interconnecting these two ring elements. The nomenclature, xe2x80x9crungsxe2x80x9d, derives from the topology of birdcage coils: when mapped to a plane, the birdcage coil is recognized as a ladder type of LC network.
Birdcage resonators of different types have been produced, depending on the desired features. Capacitors are inserted either in the rungs for a low-pass coil, or in the rings for a high-pass coil. In the present work, xe2x80x9ccoilxe2x80x9d and xe2x80x9cresonatorxe2x80x9d will generally be used interchangeably, reflecting a typically integral LC structure of the object.
The N rungs (N greater than 1) of the birdcage coil, being equally spaced about the azimuthal direction of the coil, support RF phases separated by equal increments. In an extreme example, the number of rungs has been increased substantially to form a so-called millipede coil in order to increase field homogeneity. U.S. Pat. No. 6,285,189 B1.
Many previous birdcage coils have been constructed from rungs made of copper sheet or foil with its plane perpendicular to the radial direction. This was not only due to the ease of construction but also in order to maximize the space available for a sample in view of the inner radius of the radio frequency shield. In the present invention it has been found that the cross sectional dimensions of the rungs of the birdcage coil effect the resulting Q of the coil. The filling factor of the coil (a volumetric measure of the spatial capacity for a sample) is directly effected and a lower value for filling factor diminishes the realizable signal-to-noise ratio for the NMR apparatus.
In passing, it is noted that previous NMR resonators of solenoidal geometry have employed conductors having cross sectional dimensions a, b, such that a and b are similar. Certain such solenoidal resonators, purposely so constructed, have been found to exhibit significantly enhanced Q. U.S. Pat. No. 6,087,832.
Most previous works on birdcage coil design paid relatively little attention to the unloaded coil Q. This is reasonable if the coil is to be used for the imaging of relatively large (tens of cm) lossy biological samples since 1/Qtotal=1/Qsample+1/Qcoil where Qtotal, Qsample and Qcoil are respectively the total Q factor, the Q factor of the sample and the Q factor of the coil and Qsample less than  less than Qcoil in most such applications. If the coil and sample size decreases, however, the coil Q factor starts to strongly affect the performance because Qsample and Qcoil vary proportionally to Lxe2x88x924 and L, respectively, where L is the length scale of the coil and sample.
It is therefore an advantage of this invention to provide a birdcage resonator coil designed to maximize its Q factor by optimizing the gap and rung dimensions.
This invention provides a method of producing a birdcage resonator coil by properly designing the cross-sectional shape of its rungs.
It is yet another advantage of the invention to extend the foregoing to related resonant coil structures.
A birdcage resonator coil embodying this invention, with which the above and other objects can be achieved, may be characterized as comprising a pair of conductor rings separated from each other along a central axis and a plural number of linearly elongated conductor rungs which extend between the rings and are spaced azimuthally around the central axis, wherein each of the rungs is cross-sectionally so dimensioned that its radial extension (thickness) is at least greater than its azimuthal extension (width). Among other dependencies of Q, a lower resistance for the rungs (inductive elements) leads to a higher Q. Although a larger cross section for these inductive elements will reduce the resistance, the filling factor may be compromised by the intrusion of the radial component of the enlarged cross section. The radial extension outwardly promotes further RF losses on a surrounding shield. As the azimuthal component of the inductor cross section increases, the spatial interval between rungs decreases and the constraints on the B field of the coil lead to local distortion of that field. A more important consideration is that the localized regions of high field imply localized regions of intense surface current density for proximate conductors and consequent high Ohmic losses. Thus, Q is degraded for birdcage coils as the spatial interval between rungs is reduced. It can be ascertained that the Q factor can be thereby improved without significantly affecting its filling factor by solving Maxwell""s equations numerically in two dimensions for the magnetic vector potential, from which the magnetic field distribution can be derived for a transverse section of the interior volume of the coil. The geometric properties of the conductor dimensions effecting Q may be reconciled with field distribution to obtain a satisfactory compromise for coil design. Alternatively, an NMR resonator may be in the form of a so-called saddle coil comprising a wire flattened to present a sufficient width and thickness consistent with the principles described herein for optimization of birdcage resonators.